Men who have early-stage prostate cancer will live just as long if they forgo surgery, which carries risks of impotence and incontinence, and simply do nothing but watch their cancer, according to the results of the largest clinical trial on the issue to date.

The findings, published in Thursday’s issue of the New England Journal of Medicine, fall in line with mounting concerns about the overscreening and overtreatment of prostate tumors in the U.S. Each year, millions of men are screened for early disease and, based on the results, many undergo invasive biopsies, surgery or other cancer treatment for tumors that would not have killed them anyway. The procedures may cause impotence, incontinence and even death; as many as 1,000 to 1,300 men die due to complications associated with treatments prompted by screening.

In May, the U.S. Preventive Services Task Force (USPSTF) recommended against routine screening for prostate cancer using the PSA, or prostate-specific antigen, test. The decision was a controversial one, given that the PSA test has become an entrenched part of men’s health care over the last two decades. The task force’s recommendation wasn’t the last word on prostate-cancer care, and despite the results of the new study, which is being hailed as a landmark trial, the debate will likely continue.

The Prostate Cancer Intervention Versus Observation Trial, or PIVOT, included 731 men with early-stage prostate cancer, about half of whom detected their cancer through a PSA test. Men were randomly assigned to treat their cancer with prostate-removal surgery — known as radical prostatectomy — or to undergo “watchful waiting,” in which men monitored their tumor without treatment unless it showed signs of progressing.

By the end of the 15-year study, about half the men (354) had died, the vast majority from causes unrelated to prostate cancer. The researchers found no statistical difference in mortality between the surgery and watchful-waiting groups: 171 out of the 364 men assigned to surgery died, compared with 183 out of 367 men assigned to observation.

During the study period, just 52 men (7%) died of prostate cancer, and again, there was no difference in death rate between the two study groups.

Still, the findings do not suggest that surgery offers no survival benefit at all. In a subgroup of men with high-risk early-stage cancers — those with high PSA scores, over 10 nanograms per milliliter of blood — surgery did lead to a slight advantage. Overall, among men with high PSA scores, there were 13% fewer deaths in the surgery group than in the watchful-waiting group, and there were 7% fewer deaths specifically related to prostate cancer. These men were also half as likely to have their cancer spread to their bones if they had surgery.

The study included only men with early-stage prostate cancer (localized to the prostate) — which accounts for 4 out of 5 prostate cancers — so the results do not apply to those with advanced disease. The study also included mostly older men (average age 67), so it’s not clear how the findings may be relevant to younger patients. But because PSA tests catch tumors so early these days, the authors think that men who are diagnosed today — as compared to 1994 when the study began — have a much better prognosis with simple observation.

As the study authors conclude:

Our findings support observation for men with localized prostate cancer, especially those who have a low PSA value and those who have low-risk disease. Up to two thirds of men who have received a diagnosis of prostate cancer have a low PSA value or low-risk disease, but nearly 90% receive early intervention — typically surgery or radiotherapy.

About 242,000 men will be diagnosed with prostate cancer this year, according to the American Cancer Society, thanks largely to PSA screening, and about 28,000 men will die of the disease. That makes prostate cancer the second-leading cause of cancer death in men, after lung cancer. But increasingly the data show that early detection with PSA screening has little impact on men’s risk of death. Prostate cancer is typically so slow growing that most doctors say many men die with prostate cancer, not from it. One large study, the Prostate Cancer Prevention Trial, showed that while 28% of men in their 60s were diagnosed with prostate cancer after registering high levels of PSA, only 3% of those men ultimately died of the disease. In the current study as well, the vast majority of men who died did not die of prostate cancer.

Such findings suggest that doctors are detecting many cases of prostate cancer that men would actually be better off never knowing about. “We formulated this dogma that all cancer needs to be detected early and that treatment was our only hope decades ago when our ability to get in and find asymptomatic disease was limited and almost negligible. At that time, the assumption was that any cancer we found was the bad kind,” Dr. Michael LeFevre, co–vice chair of the USPSTF, told Healthland in May. “But the more technology advances to detect asymptomatic disease, the more we learn that not all of it needs to be detected.”

What cancer doctors really need is a better way to screen for and identify prostate tumors that will be aggressive and will therefore benefit from aggressive treatment. The widely used PSA test isn’t sophisticated enough to do that.

Based on the current evidence, the study’s authors hope that more men will be encouraged to consider watchful waiting before rushing to treat with surgery or radiation, which about 90% of patients with early-stage cancer end up doing. Some 100,000 to 120,000 men undergo radical prostatectomy each year.

“When most men are told they have prostate cancer, their immediate thought is, ‘Oh, my God, I’m going to die,’ and their immediate next step is, ‘Let’s do something about this,'” Dr. Durado Brooks, director of prostate and colorectal cancers for the American Cancer Society, told the Los Angeles Times. By then, “the idea of an observation approach is lost,” he said.